In general, a digital image is expressed in the form in which noise was attached to an original image by an image system or the like in a process in which the digital image is acquired as analog signals and a process in which the digital image is converted into digital signals. An image distorted by the attached noise may be a factor of a reduction in compression efficiency due to an increase of high-frequency component in an image compressing process, and may be a factor of a reduction in a recognition rate in an image recognition system. The image distortion phenomenon due to the attached noise may cause problems in various image application fields, and therefore it is very important to effectively minimize the attached noise in a pre-processing process in order to maximize the performance of the corresponding application system.
Meanwhile, the ultimate goal of noise removal techniques is to generate a restored image close to the original image while preserving detailed information and boundaries, and various related techniques have been proposed. Noise generated in the acquisition process of most image systems has Gaussian distribution characteristics, and as the representative technique concerning such noise characteristics, a least mean square (LMS) technique has been used. The above-described technique is accompanied by an image contour degradation phenomenon in a filtering process. In the representative technique based on local statistical properties, an average filter and a median filter have been used, and weight-based local media filter technique that is a modified type has been studied, but has a limit in the minimization of the degradation phenomenon.
In order to minimize the degradation phenomenon, various methods to give local adaptability have been studied. As the representative method, a two-way filter technique in which differences of local statistical characteristics and radiation analysis between pixels are utilized in the weight of the filter has been studied. However, the two-way filter technique is significantly effective for maintaining a contour component of a restored image compared to an existing technique, but is unsatisfactory for a noise detection process for determining the presence and absence of noise, and therefore the two-way filter technique has a limitation in the performance improvement when an amount of the attached noise is small.